Let’s see it’s pros and cons, alright?
-High Special Attack stat of 120
-Good Defense stat of 115
- 3 of it’s ability are not bad
-Attack stat was good also which means that it could also be a Psychical Pokemon
-Good defensive styling of Steel / Dragon, which gains it 10 Resistence, 1 immunities and only 2 weakness
-50 Special Defense, which means that could OHKO by neutral special damage
-Movepool wasn’t as good as you said
-85 Speed was not good in trick room or no trick room, which needs a spes or else will be outspeed and OHKO by the others
-Was outclassed by the other Special Dragon type
-No good coverage moves for Fighting and Ground types, which was bad
So, the main reason why the Smogon put it in NU was because that it got bad Special defense, not good speed, no coverage movepool and was outclassed by the other dragon types.
Hope it helps!